This invention relates to a centrifugal compressor and, in particular, to a hybrid bearing arrangement for a centrifugal compressor.
Many centrifugal compressors, such as those employed in refrigeration systems, are driven by electric motors. The motor shaft is typically connected to the rotor shaft of the compressor which supports one or more impeller wheels by means of a gear train. The gear train is designed to increase the rotor speed above that of the motor shaft to obtain optimum efficiency. In refrigeration applications, single stage centrifugal compressors are generally favored because this type of single stage machine produces a large pressure ratio for the single stage of compression and it is more easily manufactured when compared to the more complex multi-stage machines.
Generally, the low speed motor shaft and the high speed impeller shaft are supported by oil lubricated hydrodynamic journal or sleeve bearings. In addition to radial loads exerted upon the shafts, gas pressures and the gearing arrangement also create relatively high axial loads upon the shafts. These axial loads require the further use of oil lubricated hydrodynamic thrust bearings to help carry the extra loading on the shafts.
Hydrodynamic bearings have higher friction losses when compared to ball and roller bearings, particularly at startup. To reduce friction losses and prevent damage at startup and coast down, hydrodynamic bearings are equipped with soft metal linings that are about 0.020xe2x80x3 to 0.040xe2x80x3 thick. In the case of an impeller shaft, the clearance between the impeller blades and the shroud that encompasses the impeller is oftentimes less than the thickness of the protective liner. In the case when there is damage or wear to thrust bearings that support the impeller shaft, the impeller can be displaced significantly with the shaft whereupon the impeller blades will rub against the shroud resulting in serious damage being done to the impeller. Although hydrodynamic bearings have higher power losses when compared to roller and ball bearings, they are better able to dampen out troublesome vibrations when compared to other bearings.
It is, therefore, an object of the present invention to improve centrifugal compressors and, in particular, single stage centrifugal compressors of the type employed in air conditioning and refrigeration systems.
It is a further object of the present invention to protect the impeller of a single stage centrifugal compressor from damage.
It is still a further object of the present invention to enhance the performance of a single stage centrifugal compressor of the type employed in an air conditioning or refrigeration system.
Another object of the present invention is to increase the bearing reliability of a centrifugal compressor.
Yet another object of the present invention is to reduce bearing frictional losses in a centrifugal compressor.
These and other objects of the present invention are attained by a centrifugal compressor having a low speed shaft connected to a drive motor and a high speed shaft coupled to the low speed shaft by a gear train. An impeller is mounted upon the high speed shaft and the shaft is supported upon ball and roller bearings which reduce friction losses within the machine and prevent rubbing damage to the impeller due to axial loading of the shaft. The low speed shaft is supported by oil lubricated hydrodynamic bearings which dampen vibrational stresses induced in both the interconnected low speed shaft and high speed shafts.